FIG. 5 is a sectional view showing a rotary encoder in a conventional wheel rolling bearing assembly. A wheel rolling bearing assembly 210 includes: an outer ring 210a fixed to a vehicle body via an unillustrated knuckle; an inner ring 210b fixed to an unillustrated axle shaft mounted with an unillustrated wheel; a plurality of balls 210c as a rolling element interposed between the outer ring 210a and the inner ring 210b; and a cage 210d formed with pocket portions at regular space intervals, the pocket portions receiving the balls 210c. 
The rotary encoder for detecting the number of revolutions of the wheel rolling bearing assembly 210 includes: a rotor 220 fixed to the wheel rolling bearing assembly 210; and a rotation detection sensor S for magnetically detecting the rotation of the rotor 220.
The rotor 220 is formed from rubber admixed with magnetic powder, such as ferrite powder, and in an annular shape. The rotor includes: a magnetized portion 222 circumferentially magnetized in alternating S-polarity and N-polarity; and an annular core portion 221 supporting the magnetized portion 222.
The core portion 221 further includes: a cylindrical portion 221a press-fitted on an outer periphery of a shoulder 210e of the inner ring 210b; and a flange portion 221b extended radially outwardly from one axial end of the cylindrical portion 221a. The core portion 221 is formed in an L-shape in section.
The magnetized portion 222 is fixed to a lateral side of the flange 221b so as to oppose a detecting portion of the rotation detection sensor S in an axial direction.
When the rotor 220 so fixed to the inner ring 210b is rotated in conjunction with the rotation of the wheel, the polarities of the magnetized portion 222 opposite the rotation detection sensor S alternate with each other, so that the rotation detection sensor S outputs the changes of magnetic field as the number of revolutions of the wheel.    Patent Document 1: Japanese Unexamined Patent Publication No. 2003-75194